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The BMW 3.0CS/i/L, 2800CS, and the 2500CS all share the same body style. BMW names its models based on the body. This particular chassis has the designation E9. So, to cover all the possible models, this kind of car is usually referred to as an E9.
There were several versions of coupes using the E9 chassis: 2800CS, 3.0CS, 3.0CSi, 3.0CSL and 2500CS. The numbers denote the displacement of the engine (2800=2.8L, 3.0=3.0L, etc) and all cars share the same basic engine (M30) which is a single overhead cam straight six with displacements varying from 2.5 to 3.5 liters. CS means "Coupe-Sport". The other letters, or lack of letters, indicate what kind of CS it is. A regular CS designation, without any additional letters, uses two down-draft Zenith carbs. A CS with an "i" after it is a fuel injected version, only officially sold outside the US. They run on an analog fuel injection system called D-Jet. A 3.0CSL is a lightweight version of the original coupe, with aluminum doors, hood, and trunk, as well as thinner steel for the rest of the body, manual steering, plexiglass windows, no bumpers, and a special interior. All these factors made the original CSLs weigh in at barely 2500lbs. However, since most customers didn't want a 10k$ car without these things, most CSLs have what is known as the "city pack", which includes just about everything that was removed, but keeps the aluminum trunk and hood, as well as the interior. Even with everything added again, the "city pack" CSL weighs in at about 2800lbs.
The production run lasted from 1968 to 1976. The body started as the 2800CS and ended with the 2.5CS. Approximately 30,000 E9s were made over the entire production run. Official US cars came entirely through Max Hoffman, as there was no BMW NA at the time. Official US models were made from 1970 to 1974.
The car had a base price of ~10kUS when launched and the base model had quite a few options from the start. The price eventually reached 16k US in 1974. Buyers could add several nice options, including:
  • Leather seats
  • Air Conditioning
  • Sunroof (later motorized)
  • Power windows (later standard, rears always power, fronts could be manual)
  • Radio
  • Limited Slip Differential (25% lockup)
  • Metallic paint
  • Most US coupes were fully optioned, aside from the Limited Slip Differential.
There are major differences in US and Euro E9s. First, the compression ratio in the US 3.0CS was reduced to 8:1, while the Euro cars were 9:1, giving them more power. The US cars have side marker lights in the front and back, which go next to the main turn signals. The later (>73) US cars have additional smog equipment, such as an EGR system. Starting in 1974 the US versions came with large 5mph aluminum bumpers front and rear. Although these large bumpers are said to take away from the beautiful lines of the coupe, the 5mph crash bumpers work well and have saved a number of coupes over the years. Some owners prefer the later large bumpered cars if they do a lot of driving in the city or traffic. Later US coupes also have a "Fasten Seat Belt" light pod on the dash. The Euro gauges are in Metric, while the US ones are in standard. The heater vent controls are either in German or in the international symbols for Euro cars, while they are in English for the US cars. Euro cars also only had a lap belt, while US cars have stock shoulder belts. This is usually already fixed, as most of the Euro cars have shoulder belts added later on, either as part of the federalization process or the owner added them.

The US also had slightly larger front turn signal lenses. A slight difference in Euro models can be seen in Italian coupes, which have a white and amber turn signal lens, rather than the all amber used everywhere else. French cars have yellow headlight lenses and amber reverse lights. All US models have DOT required spacers on top of R/L strut below inner engine compartment sheet metal to raise front end height approximately 1" to meet sealed beam headlight height requirements. Early cars had cloissoine roundels at "C' pillar and chromed brass trim between tailights which was changed to raised painted alum. roundels and anodized alum. during 1972 model run to reduce costs. Early cars had wider stock seats and seat back release mechanism at the hinge, by 1971 a small lever mounted on outside of upper seat edge replaced the old mechanism.
The main changes over the production runs of the standard coupes consist of the addition of the large 5mph bumpers for US cars and introducing new models, such as the injected 3.0's and the 2.5CS. The CSL had a variety of changes over its run, but since it is not a standard coupe, it will be discussed below.
More often than not, coupes can be found in the normal places. Ebay is a usual starting point, a variety of classic car trader sites and craigslist.com is becoming more and more popular. However, one of the best places to find a coupe is in the classifieds section of this site. The majority of the owners are knowledgable about coupes and generally take better care of them.
Prices for E9 coupes vary from less than $1k to more than $100k, so it can be a challenge figuring out if a coupe is priced correctly. The deciding factor is usually how solid the chassis is in regards to rust. Here's a quick reference for what to expect in the different price ranges:

$5k and below: A rust bucket and considered a parts car. There are some out there that are worth keeping in this range, but they're getting really hard to find.

$5k to $10k: This price range typically has cars that may look good from a distance but show lots of problems when you start checking them out closely and will need full restoration. Coupes are very good cars mechanically, so you'll also find cars in this range that run great but have some major rust issues.

$10k to $15k: Daily driver range but will still have rust issues. These cars are typically good looking cars and get lots of positive attention while on the road, although they still need to be inspected very carefully for rust problems. There are a good number of cars out there on the market owned by people that don't know much about them but had the money to buy one. You may find rust issues that they didn't know about or chose to ignore thinking it wasn't a big deal.

$15k to 25k: Cars in this range used to be very nice the market has changed and these will still have issues to address. Inspection might find a small crack in the dash, a split seams or two in the seats and a few imperfections in the paint, but for the most part the car should be well sorted out. There should be few signs of rust problems on the car. You might also expect to see signs of rust repair in the past. If so, try to gauge how well the work was done. If the car has been repainted in the past (most have), ask how long ago the job was done. If the paint was done over three or four years prior to inspection, bad repairs would start showing problems. A fairly new paint job might not show poor rust repair work until a couple years after you buy the car.

$25k and above: Little or no problems to speak of. Some may have modifications done to the drivetrain, which if done correctly and tastefully, does not adversely affect the value of the car. In some cases it can increase value if its a popular upgrade. Keep in mind, BMW's in general are popular cars for enthusiasts to modify and the E9 coupe is no exception.

CSL's: Used to be considered a fancy CS, but now they sit in a league of their own. Within the last few years, some nicely restored CSL's have sold at auction for well over $100k. Other than low production numbers and a racing history, they're not really that much different than a typical CS or CSi.
The mechanicals for the car are still available from BMW or are (for the most part) interchangeable with a later car, with that part still being made by BMW. Unfortunately, the mechanicals are the easy part. If you find a coupe with no drivetrain but a perfect body and interior for less than 15k, you got a deal. The entire drivetrain can be had from wreckers across the world for about 2000$ (US) if you use used components. The interior bits are almost all no longer available, so everything that needs replacing there requires a parts car or a very expensive phone call to one of the few E9 wreckers around the world. Body panels are similar. Most body panels now are taken from parts cars, so they can have their share of dings before it ever gets on the car. As long as the body and interior are kept in good condition, the car will run pretty much forever. The M30 has been known to go for more than 300k miles between rebuilds. Aside from periodic (15k) valve adjustments, the engine is pretty darn bulletproof. Timing chains typically last the life of the engine, and hardly ever need to be replaced.
The best place to start is in the Links section of this website.
Pretty much everything is upgradeable since the car is from the 70s. No funky computer control to mess around with. For more about this, check out the Upgrade FAQ.
Mechanically, the cars are very durable, although the M30 can be prone to blowing head gaskets. This is largely due to the lack of cooling on the stock E9 for hot climates. The car was designed for the far more moderate European climate, hence the trouble with high ambient heat. It is possible to somewhat remedy this by upgrading the original fan and the original fan clutch. The original fan is a 5 blade design, which can be easily replaced with a 9 blade design if the fan clutch is also updated. Recoring the radiator is another option that is very helpful. Other than that, the most common failure point is the body. Again, rust is a coupe killer.
BMW mobile tradition maintains records on all the coupes produced, so if you can get the VIN, you can email them and they can tell you a bit about the car. They can usually tell you when the car was made, when it was sold, what kind it originally was, (i/L/carbed) and the original color. The original color can also usually be found on a tag on one of the door jambs, on the side closest to the engine bay. It says "Karmann" and has the paint code on it. Also, although this is somewhat rare to find now, coupes were supposed to have a build sheet attached to the back of the rear seats. The build sheet detailed what options were built into the car, the date produced, and a host of other info.
The VIN is located in at least three places: the heater bulkhead in the engine bay, the ring around the starter motor on the engine, and the factory stamped plate, also in the engine bay. US cars have the VIN stamped on the metal steering column cover. The VIN is also sometimes shown on a small tab, usually interted in the windshield so it is visible from the outside, much like a modern car. Although decidedly less visible, the VIN is also stamped on the transmission case. The VIN ranges for all coupes are located here: